The normal way to run a Perl program is by making it directly executable,
or else by passing the name of the source file as an argument on the command line.
(An interactive Perl environment is also possible--see perldebug for details on how to do that.) Upon startup,
Perl looks for your program in one of the following places:

Specified line by line via -e or -E switches on the command line.

Contained in the file specified by the first filename on the command line.
(Note that systems supporting the #! notation invoke interpreters this way.
See "Location of Perl".)

Passed in implicitly via standard input.
This works only if there are no filename arguments--to pass arguments to a STDIN-read program you must explicitly specify a "-" for the program name.

With methods 2 and 3,
Perl starts parsing the input file from the beginning,
unless you've specified a -x switch,
in which case it scans for the first line starting with #! and containing the word "perl",
and starts there instead.
This is useful for running a program embedded in a larger message.
(In this case you would indicate the end of the program using the __END__ token.)

The #! line is always examined for switches as the line is being parsed.
Thus,
if you're on a machine that allows only one argument with the #! line,
or worse,
doesn't even recognize the #! line,
you still can get consistent switch behaviour regardless of how Perl was invoked,
even if -x was used to find the beginning of the program.

Because historically some operating systems silently chopped off kernel interpretation of the #! line after 32 characters,
some switches may be passed in on the command line,
and some may not; you could even get a "-" without its letter,
if you're not careful.
You probably want to make sure that all your switches fall either before or after that 32-character boundary.
Most switches don't actually care if they're processed redundantly,
but getting a "-" instead of a complete switch could cause Perl to try to execute standard input instead of your program.
And a partial -I switch could also cause odd results.

Some switches do care if they are processed twice,
for instance combinations of -l and -0.
Either put all the switches after the 32-character boundary (if applicable),
or replace the use of -0digits by BEGIN{ $/ = "\0digits"; }.

Parsing of the #! switches starts wherever "perl" is mentioned in the line.
The sequences "-*" and "- " are specifically ignored so that you could,
if you were so inclined,
say

#!/bin/sh
#! -*-perl-*-
eval 'exec perl -x -wS $0 ${1+"$@"}'
if 0;

to let Perl see the -p switch.

A similar trick involves the env program, if you have it.

#!/usr/bin/env perl

The examples above use a relative path to the perl interpreter, getting whatever version is first in the user's path. If you want a specific version of Perl, say, perl5.005_57, you should place that directly in the #! line's path.

If the #! line does not contain the word "perl" nor the word "indir" the program named after the #! is executed instead of the Perl interpreter. This is slightly bizarre, but it helps people on machines that don't do #!, because they can tell a program that their SHELL is /usr/bin/perl, and Perl will then dispatch the program to the correct interpreter for them.

After locating your program, Perl compiles the entire program to an internal form. If there are any compilation errors, execution of the program is not attempted. (This is unlike the typical shell script, which might run part-way through before finding a syntax error.)

If the program is syntactically correct, it is executed. If the program runs off the end without hitting an exit() or die() operator, an implicit exit(0) is provided to indicate successful completion.

The Win95/NT installation, when using the ActiveState installer for Perl, will modify the Registry to associate the .pl extension with the perl interpreter. If you install Perl by other means (including building from the sources), you may have to modify the Registry yourself. Note that this means you can no longer tell the difference between an executable Perl program and a Perl library file.

at the top of your program, where -mysw are any command line switches you want to pass to Perl. You can now invoke the program directly, by saying perl program, or as a DCL procedure, by saying @program (or implicitly via DCL$PATH by just using the name of the program).

This incantation is a bit much to remember, but Perl will display it for you if you say perl "-V:startperl".

Command-interpreters on non-Unix systems have rather different ideas on quoting than Unix shells. You'll need to learn the special characters in your command-interpreter (*, \ and " are common) and how to protect whitespace and these characters to run one-liners (see -e below).

On some systems, you may have to change single-quotes to double ones, which you must not do on Unix or Plan 9 systems. You might also have to change a single % to a %%.

It may seem obvious to say, but Perl is useful only when users can easily find it. When possible, it's good for both /usr/bin/perl and /usr/local/bin/perl to be symlinks to the actual binary. If that can't be done, system administrators are strongly encouraged to put (symlinks to) perl and its accompanying utilities into a directory typically found along a user's PATH, or in some other obvious and convenient place.

In this documentation, #!/usr/bin/perl on the first line of the program will stand in for whatever method works on your system. You are advised to use a specific path if you care about a specific version.

#!/usr/local/bin/perl5.00554

or if you just want to be running at least version, place a statement like this at the top of your program:

specifies the input record separator ($/) as an octal or hexadecimal number. If there are no digits, the null character is the separator. Other switches may precede or follow the digits. For example, if you have a version of find which can print filenames terminated by the null character, you can say this:

find . -name '*.orig' -print0 | perl -n0e unlink

The special value 00 will cause Perl to slurp files in paragraph mode. Any value 0400 or above will cause Perl to slurp files whole, but by convention the value 0777 is the one normally used for this purpose.

You can also specify the separator character using hexadecimal notation: -0xHHH..., where the H are valid hexadecimal digits. Unlike the octal form, this one may be used to specify any Unicode character, even those beyond 0xFF. So if you really want a record separator of 0777, specify it as -0x1FF. (This means that you cannot use the -x option with a directory name that consists of hexadecimal digits, or else Perl will think you have specified a hex number to -0.)

As of 5.8.1, the -C can be followed either by a number or a list of option letters. The letters, their numeric values, and effects are as follows; listing the letters is equal to summing the numbers.

I 1 STDIN is assumed to be in UTF-8
O 2 STDOUT will be in UTF-8
E 4 STDERR will be in UTF-8
S 7 I + O + E
i 8 UTF-8 is the default PerlIO layer for input streams
o 16 UTF-8 is the default PerlIO layer for output streams
D 24 i + o
A 32 the @ARGV elements are expected to be strings encoded
in UTF-8
L 64 normally the "IOEioA" are unconditional, the L makes
them conditional on the locale environment variables
(the LC_ALL, LC_TYPE, and LANG, in the order of
decreasing precedence) -- if the variables indicate
UTF-8, then the selected "IOEioA" are in effect
a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
code in debugging mode.

For example, -COE and -C6 will both turn on UTF-8-ness on both STDOUT and STDERR. Repeating letters is just redundant, not cumulative nor toggling.

The io options mean that any subsequent open() (or similar I/O operations) in the current file scope will have the :utf8 PerlIO layer implicitly applied to them, in other words, UTF-8 is expected from any input stream, and UTF-8 is produced to any output stream. This is just the default, with explicit layers in open() and with binmode() one can manipulate streams as usual.

-C on its own (not followed by any number or option list), or the empty string "" for the PERL_UNICODE environment variable, has the same effect as -CSDL. In other words, the standard I/O handles and the default open() layer are UTF-8-fied but only if the locale environment variables indicate a UTF-8 locale. This behaviour follows the implicit (and problematic) UTF-8 behaviour of Perl 5.8.0. (See "UTF-8 no longer default under UTF-8 locales" in perl581delta.)

You can use -C0 (or "0" for PERL_UNICODE) to explicitly disable all the above Unicode features.

The read-only magic variable ${^UNICODE} reflects the numeric value of this setting. This variable is set during Perl startup and is thereafter read-only. If you want runtime effects, use the three-arg open() (see "open" in perlfunc), the two-arg binmode() (see "binmode" in perlfunc), and the open pragma (see open).

(In Perls earlier than 5.8.1 the -C switch was a Win32-only switch that enabled the use of Unicode-aware "wide system call" Win32 APIs. This feature was practically unused, however, and the command line switch was therefore "recycled".)

Note: Since perl 5.10.1, if the -C option is used on the #! line, it must be specified on the command line as well, since the standard streams are already set up at this point in the execution of the perl interpreter. You can also use binmode() to set the encoding of an I/O stream.

causes Perl to check the syntax of the program and then exit without executing it. Actually, it will execute and BEGIN, UNITCHECK, or CHECK blocks and any use statements: these are considered as occurring outside the execution of your program. INIT and END blocks, however, will be skipped.

runs the program under the control of a debugging, profiling, or tracing module installed as Devel::MOD. E.g., -d:DProf executes the program using the Devel::DProf profiler. As with the -M flag, options may be passed to the Devel::MOD package where they will be received and interpreted by the Devel::MOD::import routine. Again, like -M, use --d:-MOD to call Devel::MOD::unimport instead of import. The comma-separated list of options must follow a = character. If t is specified, it indicates to the debugger that threads will be used in the code being debugged. See perldebug.

sets debugging flags. To watch how it executes your program, use -Dtls. (This works only if debugging is compiled into your Perl.) Another nice value is -Dx, which lists your compiled syntax tree. And -Dr displays compiled regular expressions; the format of the output is explained in perldebguts.

As an alternative, specify a number instead of list of letters (e.g., -D14 is equivalent to -Dtls):

All these flags require -DDEBUGGING when you compile the Perl executable (but see :opd in Devel::Peek or "'debug' mode" in re which may change this). See the INSTALL file in the Perl source distribution for how to do this. This flag is automatically set if you include -g option when Configure asks you about optimizer/debugger flags.

If you're just trying to get a print out of each line of Perl code as it executes, the way that sh -x provides for shell scripts, you can't use Perl's -D switch. Instead do this

may be used to enter one line of program. If -e is given, Perl will not look for a filename in the argument list. Multiple -e commands may be given to build up a multi-line script. Make sure to use semicolons where you would in a normal program.

Perl can be built so that it by default will try to execute $Config{sitelib}/sitecustomize.pl at startup (in a BEGIN block). This is a hook that allows the sysadmin to customize how Perl behaves. It can for instance be used to add entries to the @INC array to make Perl find modules in non-standard locations.

Since it is an actual do (not a require), sitecustomize.pl doesn't need to return a true value. The code is run in package main, in its own lexical scope. However, if the script dies, $@ will not be set.

The value of $Config{sitelib} is also determined in C code and not read from Config.pm, which is not loaded.

The code is executed very early. For example, any changes made to @INC will show up in the output of `perl -V`. Of course, END blocks will be likewise executed very late.

To determine at runtime if this capability has been compiled in your perl, you can check the value of $Config{usesitecustomize}.

specifies that files processed by the <> construct are to be edited in-place. It does this by renaming the input file, opening the output file by the original name, and selecting that output file as the default for print() statements. The extension, if supplied, is used to modify the name of the old file to make a backup copy, following these rules:

If no extension is supplied, no backup is made and the current file is overwritten.

If the extension doesn't contain a *, then it is appended to the end of the current filename as a suffix. If the extension does contain one or more * characters, then each * is replaced with the current filename. In Perl terms, you could think of this as:

($backup = $extension) =~ s/\*/$file_name/g;

This allows you to add a prefix to the backup file, instead of (or in addition to) a suffix:

$ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to
# 'orig_fileA'

Or even to place backup copies of the original files into another directory (provided the directory already exists):

except that the -i form doesn't need to compare $ARGV to $oldargv to know when the filename has changed. It does, however, use ARGVOUT for the selected filehandle. Note that STDOUT is restored as the default output filehandle after the loop.

As shown above, Perl creates the backup file whether or not any output is actually changed. So this is just a fancy way to copy files:

You cannot use -i to create directories or to strip extensions from files.

Perl does not expand ~ in filenames, which is good, since some folks use it for their backup files:

$ perl -pi~ -e 's/foo/bar/' file1 file2 file3...

Note that because -i renames or deletes the original file before creating a new file of the same name, Unix-style soft and hard links will not be preserved.

Finally, the -i switch does not impede execution when no files are given on the command line. In this case, no backup is made (the original file cannot, of course, be determined) and processing proceeds from STDIN to STDOUT as might be expected.

enables automatic line-ending processing. It has two separate effects. First, it automatically chomps $/ (the input record separator) when used with -n or -p. Second, it assigns $\ (the output record separator) to have the value of octnum so that any print statements will have that separator added back on. If octnum is omitted, sets $\ to the current value of $/. For instance, to trim lines to 80 columns:

perl -lpe 'substr($_, 80) = ""'

Note that the assignment $\ = $/ is done when the switch is processed, so the input record separator can be different than the output record separator if the -l switch is followed by a -0 switch:

-Mmodule executes usemodule; before executing your program. You can use quotes to add extra code after the module name, e.g., '-MMODULE qw(foo bar)'.

If the first character after the -M or -m is a dash (-) then the 'use' is replaced with 'no'.

A little builtin syntactic sugar means you can also say -mMODULE=foo,bar or -MMODULE=foo,bar as a shortcut for '-MMODULE qw(foo bar)'. This avoids the need to use quotes when importing symbols. The actual code generated by -MMODULE=foo,bar is use module split(/,/,q{foo,bar}). Note that the = form removes the distinction between -m and -M.

A consequence of this is that -MMODULE=number never does a version check, unless MODULE::import() itself is set up to do a version check, which could happen for example if MODULE inherits from Exporter.

causes Perl to assume the following loop around your program, which makes it iterate over filename arguments somewhat like sed -n or awk:

LINE:
while (<>) {
... # your program goes here
}

Note that the lines are not printed by default. See "-p" to have lines printed. If a file named by an argument cannot be opened for some reason, Perl warns you about it and moves on to the next file.

Also note that <> passes command line arguments to "open" in perlfunc, which doesn't necessarily interpret them as file names. See perlop for possible security implications.

Here is an efficient way to delete all files that haven't been modified for at least a week:

find . -mtime +7 -print | perl -nle unlink

This is faster than using the -exec switch of find because you don't have to start a process on every filename found. It does suffer from the bug of mishandling newlines in pathnames, which you can fix if you follow the example under -0.

BEGIN and END blocks may be used to capture control before or after the implicit program loop, just as in awk.

If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to the next file. Note that the lines are printed automatically. An error occurring during printing is treated as fatal. To suppress printing use the -n switch. A -p overrides a -n switch.

BEGIN and END blocks may be used to capture control before or after the implicit loop, just as in awk.

enables rudimentary switch parsing for switches on the command line after the program name but before any filename arguments (or before an argument of --). Any switch found there is removed from @ARGV and sets the corresponding variable in the Perl program. The following program prints "1" if the program is invoked with a -xyz switch, and "abc" if it is invoked with -xyz=abc.

#!/usr/bin/perl -s
if ($xyz) { print "$xyz\n" }

Do note that a switch like --help creates the variable ${-help}, which is not compliant with use strict "refs". Also, when using this option on a script with warnings enabled you may get a lot of spurious "used only once" warnings.

makes Perl use the PATH environment variable to search for the program unless the name of the program contains path separators.

On some platforms, this also makes Perl append suffixes to the filename while searching for it. For example, on Win32 platforms, the ".bat" and ".cmd" suffixes are appended if a lookup for the original name fails, and if the name does not already end in one of those suffixes. If your Perl was compiled with DEBUGGING turned on, using the -Dp switch to Perl shows how the search progresses.

Typically this is used to emulate #! startup on platforms that don't support #!. It's also convenient when debugging a script that uses #!, and is thus normally found by the shell's $PATH search mechanism.

This example works on many platforms that have a shell compatible with Bourne shell:

The system ignores the first line and feeds the program to /bin/sh, which proceeds to try to execute the Perl program as a shell script. The shell executes the second line as a normal shell command, and thus starts up the Perl interpreter. On some systems $0 doesn't always contain the full pathname, so the -S tells Perl to search for the program if necessary. After Perl locates the program, it parses the lines and ignores them because the variable $running_under_some_shell is never true. If the program will be interpreted by csh, you will need to replace ${1+"$@"} with $*, even though that doesn't understand embedded spaces (and such) in the argument list. To start up sh rather than csh, some systems may have to replace the #! line with a line containing just a colon, which will be politely ignored by Perl. Other systems can't control that, and need a totally devious construct that will work under any of csh, sh, or Perl, such as the following:

If the filename supplied contains directory separators (and so is an absolute or relative pathname), and if that file is not found, platforms that append file extensions will do so and try to look for the file with those extensions added, one by one.

On DOS-like platforms, if the program does not contain directory separators, it will first be searched for in the current directory before being searched for on the PATH. On Unix platforms, the program will be searched for strictly on the PATH.

Like -T, but taint checks will issue warnings rather than fatal errors. These warnings can now be controlled normally with no warnings qw(taint).

Note: This is not a substitute for -T! This is meant to be used only as a temporary development aid while securing legacy code: for real production code and for new secure code written from scratch, always use the real -T.

turns on "taint" so you can test them. Ordinarily these checks are done only when running setuid or setgid. It's a good idea to turn them on explicitly for programs that run on behalf of someone else whom you might not necessarily trust, such as CGI programs or any internet servers you might write in Perl. See perlsec for details. For security reasons, this option must be seen by Perl quite early; usually this means it must appear early on the command line or in the #! line for systems which support that construct.

This switch causes Perl to dump core after compiling your program. You can then in theory take this core dump and turn it into an executable file by using the undump program (not supplied). This speeds startup at the expense of some disk space (which you can minimize by stripping the executable). (Still, a "hello world" executable comes out to about 200K on my machine.) If you want to execute a portion of your program before dumping, use the dump() operator instead. Note: availability of undump is platform specific and may not be available for a specific port of Perl.

allows Perl to do unsafe operations. Currently the only "unsafe" operations are attempting to unlink directories while running as superuser and running setuid programs with fatal taint checks turned into warnings. Note that warnings must be enabled along with this option to actually generate the taint-check warnings.

Additionally, extra colons can be used to control formatting. A trailing colon suppresses the linefeed and terminator ";", allowing you to embed queries into shell commands. (mnemonic: PATH separator ":".)

prints warnings about dubious constructs, such as variable names mentioned only once and scalar variables used before being set; redefined subroutines; references to undefined filehandles; filehandles opened read-only that you are attempting to write on; values used as a number that don't look like numbers; using an array as though it were a scalar; if your subroutines recurse more than 100 deep; and innumerable other things.

This switch really just enables the global $^W variable; normally, the lexically scoped use warnings pragma is preferred. You can disable or promote into fatal errors specific warnings using __WARN__ hooks, as described in perlvar and "warn" in perlfunc. See also perldiag and perltrap. A fine-grained warning facility is also available if you want to manipulate entire classes of warnings; see warnings or perllexwarn.

tells Perl that the program is embedded in a larger chunk of unrelated text, such as in a mail message. Leading garbage will be discarded until the first line that starts with #! and contains the string "perl". Any meaningful switches on that line will be applied.

All references to line numbers by the program (warnings, errors, ...) will treat the #! line as the first line. Thus a warning on the 2nd line of the program, which is on the 100th line in the file will be reported as line 2, not as line 100. This can be overridden by using the #line directive. (See "Plain Old Comments (Not!)" in perlsyn)

If a directory name is specified, Perl will switch to that directory before running the program. The -x switch controls only the disposal of leading garbage. The program must be terminated with __END__ if there is trailing garbage to be ignored; the program can process any or all of the trailing garbage via the DATA filehandle if desired.

The directory, if specified, must appear immediately following the -x with no intervening whitespace.

A list of directories in which to look for Perl library files before looking in the standard library and the current directory. Any architecture-specific directories under the specified locations are automatically included if they exist, with this lookup done at interpreter startup time.

If PERL5LIB is not defined, PERLLIB is used. Directories are separated (like in PATH) by a colon on Unixish platforms and by a semicolon on Windows (the proper path separator being given by the command perl -V:path_sep).

When running taint checks, either because the program was running setuid or setgid, or the -T or -t switch was specified, neither PERL5LIB nor PERLLIB is consulted. The program should instead say:

Command-line options (switches). Switches in this variable are treated as if they were on every Perl command line. Only the -[CDIMUdmtwW] switches are allowed. When running taint checks (either because the program was running setuid or setgid, or because the -T or -t switch was used), this variable is ignored. If PERL5OPT begins with - T, tainting will be enabled and subsequent options ignored. If PERL5OPT begins with -t, tainting will be enabled, a writable dot removed from @INC, and subsequent options honored.

A space (or colon) separated list of PerlIO layers. If perl is built to use PerlIO system for IO (the default) these layers affect Perl's IO.

It is conventional to start layer names with a colon (for example, :perlio) to emphasize their similarity to variable "attributes". But the code that parses layer specification strings, which is also used to decode the PERLIO environment variable, treats the colon as a separator.

An unset or empty PERLIO is equivalent to the default set of layers for your platform; for example, :unix:perlio on Unix-like systems and :unix:crlf on Windows and other DOS-like systems.

The list becomes the default for all Perl's IO. Consequently only built-in layers can appear in this list, as external layers (such as :encoding()) need IO in order to load them!. See "open pragma" for how to add external encodings as defaults.

Layers it makes sense to include in the PERLIO environment variable are briefly summarized below. For more details see PerlIO.

A layer which does CRLF to "\n" translation distinguishing "text" and "binary" files in the manner of MS-DOS and similar operating systems. (It currently does not mimic MS-DOS as far as treating of Control-Z as being an end-of-file marker.)

A pseudolayer that manipulates other layers. Applying the :raw layer is equivalent to calling binmode($fh). It makes the stream pass each byte as-is without translation. In particular, both CRLF translation and intuiting :utf8 from the locale are disabled.

Unlike in earlier versions of Perl, :raw is not just the inverse of :crlf: other layers which would affect the binary nature of the stream are also removed or disabled.

This layer provides a PerlIO interface by wrapping system's ANSI C "stdio" library calls. The layer provides both buffering and IO. Note that the :stdio layer does not do CRLF translation even if that is the platform's normal behaviour. You will need a :crlf layer above it to do that.

A pseudolayer that enables a flag in the layer below to tell Perl that output should be in utf8 and that input should be regarded as already in valid utf8 form. WARNING: It does not check for validity and as such should be handled with extreme caution for input, because security violations can occur with non-shortest UTF-8 encodings, etc. Generally :encoding(utf8) is the best option when reading UTF-8 encoded data.

On Win32 platforms this experimental layer uses native "handle" IO rather than a Unix-like numeric file descriptor layer. Known to be buggy in this release (5.14).

The default set of layers should give acceptable results on all platforms

For Unix platforms that will be the equivalent of "unix perlio" or "stdio". Configure is set up to prefer the "stdio" implementation if the system's library provides for fast access to the buffer; otherwise, it uses the "unix perlio" implementation.

On Win32 the default in this release (5.14) is "unix crlf". Win32's "stdio" has a number of bugs/mis-features for Perl IO which are somewhat depending on the version and vendor of the C compiler. Using our own crlf layer as the buffer avoids those issues and makes things more uniform. The crlf layer provides CRLF conversion as well as buffering.

This release (5.14) uses unix as the bottom layer on Win32, and so still uses the C compiler's numeric file descriptor routines. There is an experimental native win32 layer, which is expected to be enhanced and should eventually become the default under Win32.

The PERLIO environment variable is completely ignored when Perl is run in taint mode.

On Win32 ports only, may be set to an alternative shell that Perl must use internally for executing "backtick" commands or system(). Default is cmd.exe /x/d/c on WindowsNT and command.com /c on Windows95. The value is considered space-separated. Precede any character that needs to be protected, like a space or backslash, with another backslash.

Note that Perl doesn't use COMSPEC for this purpose because COMSPEC has a high degree of variability among users, leading to portability concerns. Besides, Perl can use a shell that may not be fit for interactive use, and setting COMSPEC to such a shell may interfere with the proper functioning of other programs (which usually look in COMSPEC to find a shell fit for interactive use).

Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked when running external commands. It is recommended that you explicitly set (or delete) $ENV{PERL5SHELL} when running in taint mode under Windows.

Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers). Perl normally searches for an IFS-compatible LSP because this is required for its emulation of Windows sockets as real filehandles. However, this may cause problems if you have a firewall such as McAfee Guardian, which requires that all applications use its LSP but which is not IFS-compatible, because clearly Perl will normally avoid using such an LSP.

Setting this environment variable to 1 means that Perl will simply use the first suitable LSP enumerated in the catalog, which keeps McAfee Guardian happy--and in that particular case Perl still works too because McAfee Guardian's LSP actually plays other games which allow applications requiring IFS compatibility to work.

Relevant only if your Perl executable was built with -DDEBUGGING, this controls the behaviour of global destruction of objects and other references. See "PERL_DESTRUCT_LEVEL" in perlhacktips for more information.

Set to "1" to have Perl resolve all undefined symbols when it loads a dynamic library. The default behaviour is to resolve symbols when they are used. Setting this variable is useful during testing of extensions, as it ensures that you get an error on misspelled function names even if the test suite doesn't call them.

(Since Perl 5.8.1.) Used to randomize Perl's internal hash function. To emulate the pre-5.8.1 behaviour, set to an integer; "0" means exactly the same order as in 5.8.0. "Pre-5.8.1" means, among other things, that hash keys will always have the same ordering between different runs of Perl.

Most hashes by default return elements in the same order as in Perl 5.8.0. On a hash by hash basis, if pathological data is detected during a hash key insertion, then that hash will switch to an alternative random hash seed.

The default behaviour is to randomize unless the PERL_HASH_SEED is set. If Perl has been compiled with -DUSE_HASH_SEED_EXPLICIT, the default behaviour is not to randomize unless the PERL_HASH_SEED is set.

If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses the pseudorandom seed supplied by the operating system and libraries.

PLEASE NOTE: The hash seed is sensitive information. Hashes are randomized to protect against local and remote attacks against Perl code. By manually setting a seed, this protection may be partially or completely lost.

(Since Perl 5.8.1.) Set to "1" to display (to STDERR) the value of the hash seed at the beginning of execution. This, combined with "PERL_HASH_SEED" is intended to aid in debugging nondeterministic behaviour caused by hash randomization.

Note that the hash seed is sensitive information: by knowing it, one can craft a denial-of-service attack against Perl code, even remotely; see "Algorithmic Complexity Attacks" in perlsec for more information. Do not disclose the hash seed to people who don't need to know it. See also hash_seed() in Hash::Util.

If your Perl was configured with -Accflags=-DPERL_MEM_LOG, setting the environment variable PERL_MEM_LOG enables logging debug messages. The value has the form <number>[m][s][t], where number is the file descriptor number you want to write to (2 is default), and the combination of letters specifies that you want information about (m)emory and/or (s)v, optionally with (t)imestamps. For example, PERL_MEM_LOG=1mst logs all information to stdout. You can write to other opened file descriptors in a variety of ways:

A translation-concealed rooted logical name that contains Perl and the logical device for the @INC path on VMS only. Other logical names that affect Perl on VMS include PERLSHR, PERL_ENV_TABLES, and SYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further in perlvms and in README.vms in the Perl source distribution.

Available in Perls 5.8.1 and later. If set to "unsafe", the pre-Perl-5.8.0 signal behaviour (which is immediate but unsafe) is restored. If set to safe, then safe (but deferred) signals are used. See "Deferred Signals (Safe Signals)" in perlipc.

Equivalent to the -C command-line switch. Note that this is not a boolean variable. Setting this to "1" is not the right way to "enable Unicode" (whatever that would mean). You can use "0" to "disable Unicode", though (or alternatively unset PERL_UNICODE in your shell before starting Perl). See the description of the -C switch for more information.

Perl also has environment variables that control how Perl handles data specific to particular natural languages; see perllocale.

Perl and its various modules and components, including its test frameworks, may sometimes make use of certain other environment variables. Some of these are specific to a particular platform. Please consult the appropriate module documentation and any documentation for your platform (like perlsolaris, perllinux, perlmacosx, perlwin32, etc) for variables peculiar to those specific situations.

Perl makes all environment variables available to the program being executed, and passes these along to any child processes it starts. However, programs running setuid would do well to execute the following lines before doing anything else, just to keep people honest: